1. Field of the Invention
The present invention relates generally to AC motor drives which convert DC to AC, and more specifically to the drive for an AC motor which will drive a pump or other active loads. A particular application is to solar water pumping systems; however, this invention is not restricted to solar systems but to any varying DC power source for the purpose of controlling AC motor systems or other loads.
2. Description of the Prior Art
An AC load can be powered from a DC source by using a converter to change DC to AC. However, because of changes in both the source and the load, it can be difficult to meet the power requirements of the load. For example, a photovoltaic solar cell array is a DC source. However, the current-voltage curve shifts under varying conditions, e.g. amount of sun. Thus at a constant voltage, the amount of current that can be drawn will vary. One application of solar power is to operate water pumps. Standard water pumps are three phase AC pumps. However, the load curve of the AC pump motor also shifts with varying conditions, e.g. water depth. Thus it can be difficult to efficiently operate an AC pump from a solar array.
Many systems that control motor drives operate on a fixed frequency and attempt to maintain a fixed DC voltage. However, attempting to operate at fixed frequency requires a constant power to the motor drive. This can take form in an elaborate DC constant voltage system or some type of DC storage systems such as a battery. For a fixed frequency at constant power the system requires a constant current-voltage relationship. This system can be quite elaborate and often times expensive for an efficiently controlled system.
Controlling motors at a fixed frequency is very demanding on the DC source voltage. If the power is to remain constant at a given frequency, then a change in DC voltage must be accompanied by a change in DC current. If the voltage decreases, the current must increase which results in a further voltage decrease until a point is reached where a shutdown must occur to prevent motor damage or increased heat or other related damage.
In general, it is desirable to operate at the maximum power point on a power curve. However, it is difficult to track power. Therefore, it is desirable to find a simpler method and apparatus to control a variable speed converter or motor drive so that an AC motor or other load may be operated from a DC source under varying source and/or load conditions.